Environmental Context. Reduction of arsenic(v) to arsenic(iii) in the environment is of interest because of the greater toxicity and mobility of the latter. It is important to know to what extent humic materials (which are ubiquitous in soils) can act as abiotic reducing agents, and what factors influence their actions.

Abstract. Inorganic arsenates were found to be reduced to arsenite by homogeneous aqueous solutions of several humic and fulvic acids. Because of the concentration dependence of the redox potentials of humics, reduction was shown to be less likely in more concentrated solutions. This was especially true in higher pH ranges, and varied with the type of material used. Ion chromatography, validated by inductively coupled plasma mass spectrometry, was used to speciate arsenic after exposure to aqueous humates and fulvates. Reduction of As(v) proceeded in the 20–60% range, depending on the humic or fulvic acid used. The fraction of arsenate that was reduced initially increased with humic concentration, but leveled off as the reduction potential decreased at higher concentrations. Re-oxidation of As(iii) in humic solutions could be achieved by extended bubbling with air. Reduction capacities of two humates tested, as measured by I₂ titration, were found to be significantly different.